In New York City, some parts of Brooklyn and Queens were mistakenly classified as vacant, according to Joseph Salvo of the city planning department. "People are saying, 'Where are these areas? Because I need an apartment,' " Salvo says.

Omissions can be costly. The federal government hands out money for things such as schools and public safety based on the count.

In New York, each person left out means $3,000 a year the city won't get, says Rachaele Raynoff, planning department spokeswoman.

Government officials may submit Census challenges between June 1, 2011, and June 1, 2013. The bureau won't do recounts, but it will add people when local governments can prove errors.

After the 2000 Census, the bureau found possible problems in 1,180 of the country's 39,000 jurisdictions. In the end, it raised the national tally by about 2,700 people.

Hidalgo County, Texas, believes 8% of its rural population was missed, and the county may sue, says Jose Garza, a lawyer hired by the county. "We don't want to get caught flat-footed at the end of the day if (an appeal) doesn't work out," he says.

In New York City, 225,000 residents, 2.6% of the population, were missed, Salvo says. The Census Bureau estimated the population at 8.4 million as of July 2009. The new count shows the population at 8.175 million.

Assemblywoman Naomi Rivera, a Bronx Democrat, cites birth and death rates as evidence of an undercount. She wants the state's congressional delegation to pressure the Census Bureau to adjust the figure, says Guillermo Martinez, Rivera's legislative director. New York City will file a challenge, Salvo says.

In Albany, N.Y., the 2010 count of 97,856 put the city just short of a federal funding milestone: 100,000 people, which would make it eligible for more federal funds, says Councilman Anton Konev, a Democrat. Albany is still deciding what to do, Mayor Jerry Jennings says.

Language barriers and mistrust of Census workers, especially by immigrants, are some of the obstacles to accuracy, the bureau's Adrienne Oneto says.

Others are also to blame, says Laurence Pizer, town clerk of Plymouth, Mass. "We've noticed an increasing reluctance on the part of all citizens to give information," he says.

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